Timing pulse generator



Jan. 4, 1966 w. o. ASHCRAFT TIMING PULSE GENERATOR 2 Sheets-Sheet 1 Filed Sept. 30, 1959 2. m mR C NH S A N ID m So u m w 56 m 2. Qz moEFzwmwEB m. mv E Q II II. nQ QQM T .65 Oh I con mzo ow =9 8 momaow w 51 & mm 9 N. 4

L QM ATTORNEY W. D. ASHCRAFT TIMING PULSE GENERATOR 6 OUT INVENTOR. WILLIAM o. ASHCRAFT BY i ATTORNEY Jan. 4, 1966 Filed Sept. 30, 1959 United States Patent ()fiiice 3,227,891 Patented Jan. 4, 1966 3,227,891 TIMING PULSE GENERATOR William D. Asher-aft, Long Beach, Calif., assignor to North American Aviation, Inc. Filed Sept. 30, 1959, Ser. No. 843,534 3 Claims. (Cl. 328-185) This invention relates to a timing pulse generator and particularly concerns apparatus for determining the midpoint of a time interval.

Where intelligence is manifested by the existence of a signal of uncertain duration, it is often necessary or desirable to provide a relatively short duration indication of such intelligence. For example, in radiant or compressional wave energy tracking systems, reception of an energy pulse will indicate the presence of a target. The duration or pulse width of the received energy signal may often vary in accordance with several different characteristics of the system or target. In some applications, and in particular for use with a digital computer, there is required a sharp pulse for an indication of the target signal. Because of the uncertain or varying duration of the target pulse, the leading and trailing edges thereof will vary insofar as the time of occurrence thereof is concerned relative to the centroid of the information pulse. This centroid or the center in time is often a point of major interest. Therefore, mere differentiation of the leading or trailing edge of the information pulse will provide an output which is spaced at some unknown interval or intervals from the centroid in time of the information or target pulse.

Accordingly, it is an object of this invention to determine the centroid of a time interval.

In carrying out the invention in accordance with one embodiment thereof there is provided a timing circuit having a variable time constant. Action of the circuit is initiated at commencement of the time interval under consideration. Upon termination of such interval the time constant of the timing cricuit is halved whereby the circuit ceases its operation at a time which occurs a fixed time interval after the centroid in time of the interval to be measured. Most conveniently the timing circuit is utilized to control the unstable period of a monostable multivibrator by providing the R-C or the resistancecapacitancer time constant thereof. The multivibrator is triggered at the beginning of a pulse having a width which specifies the time interval of interest. The time constant of the multivibrator is caused to be doubled for the duration of the input pulse, and such time constant is caused to be halved at hte end of the input pulse whereby, as can be shown mathematically, the multivibrator shifts to its stable condition at the end of a constant time interval which interval starts at the centroid of the input pulse.

A further object of this invention is to provide an indication of the centroid of a pulse of uncertain duration.

Another object of the invention is to provide a relatively short pulse having a substantially fixed chronological relation to a specified time interval.

Still another object of the invention is the provision of a substantially fixed time delay between an input pulse and an output signal indicative thereof.

These and other objects of this invention will become apparent from the following description taken in connection with the accompanying drawings in which FIG. 1 illustrates a simplified embodiment of the invention;

FIG. 1a depicts a basic waveform existing in the embodiment of FIG. 1;

FIG. 2 shows a modification of the variable timing circuit of the embodiment of FIG. 1;

FIG. 3 comprises a circuit diagram of a second embodiment of the invention;

FIG. 4 is a synchrograph of certain waveforms which exist at various points during the operation of the circuit of FIG. 3; and

FIG. 5 comprises a circuit diagram of still another embodiment of the invention.

In the drawings, like reference characters refer to like parts.

Illustrated in FIG. 1 is a substantially conventional switching circuit or monostable multivibrator which has the variable timing circuit of the invention incorporated the-rein. This variable timing circuit comprises a capacitor 10 and a pair of resistors 12 and 14 in the discharge path of the capacitor whereby the capacitor may discharge to a source of positive potential. Variation of the time constant of this timing circuit is achieved by the operation of a switch schematically depicted at 16. If the resistors 12 and 14 are of equal value, it will be seen that the resistive impedance in the discharge path of capacitor 10 is comprised substantially entirely by the resistance 14 when switch 16 is open. When the switch is closed, the resistance in the discharge path is halved since the two equal value resistors 12 and 14 connected in parallel in the discharge path provide a resistive impedance equal to half of the impedance value of either of the resistors.

The circuit arrangement is such that by a connection not shown in FIG. 1 the switch 16 is caused to be closed for the duration of the input pulse which will initiate action of the timing circuit 10, 12, 14.

The monostable multivibrator comprises first and second bi-state switching devices, here illustrated as transistors 18 and 20, each having a grounded emitter and each having the collector thereof coupled to the source of positive potential by means of resistors 22 and 24 respectively. The collector of transistor 18 is coupled to the base of transistor 20 via timing capacitor 10 and diode 26 while the collector of transistor 20 is coupled to the base of transistor 18 by means of zener diode 28. A positive input pulse 30 (see also FIG. 4) is applied to the base of transistor 18 via coupling capacitor 32.

In operation transistor 20 is normally conducting by means of a positive base current from the source of positive potential through resistors 12 and 14 (switch 16 is closed) and diode 26 and through the base and emitter electrodes of the transistor to ground. The collector output of transistor 20 is low, the zener diode 28 is therefore cut off and no positive base current is provided for transistor 18. The latter is therefore cut off. During this time switch 16 is closed whereby the resistance in the discharge path of timing capacitor 10 is equal to one-half R where R is the value of each of resistors 12 and 14.

The leading edge of positive going pulse 30 initiates the conventional multivibrator switching action by causing transistor 18 to start to conduct whereby its collector potential drops and diode 26 is back-biased to cut-off by means of the negative going potential which is transmitted via capacitor 10 to the junction 11 between the anode of diode 26 and resistor 14. With diode 26 cut off, transistor 20 goes toward cut-oif and its collector potential rises to cause the zener diode 28 to break down and a positive potential is applied to the base of transistor 18. This positive feedback continues to complete the switching action. Capacitor 10 has received a negative charge increment which holds diode 26 and transistor 20 cut off. By means not shown in FIG. 1 the switch 16 is caused to be open for the duration of the input pulse 30. Thus the capacitor is starting to discharge through the doubled impedance comprising solely resistor 14.

The time constants are chosen such that the normal unstable period of the multivibrator is substantially greater than the predetermined maximum duration of the input pulse 30. Therefore, when the input pulse 30 again goes low to cause switch 16 to close (as described below), the capacitor continues its discharge but much more rapidly than before by reason of the now halved resistive impedance in its discharge path. When the capacitor has discharged sufficiently to remove the back-bias from diode 26 the latter again conducts to cause transistor 20 to initiate conduction, the multivibrator switching action aagin occurs and the device goes back to its stable state with transistor 20 conducting and transistor 18 cut off. The negative going potential at the collector output of transistor 20 which occurs when the device goes back to its stable state occurs at a time T which can be shown mathematically to be a fixed time interval after the centroid in time of the input pulse 30 as illustrated in FIG. 4. The latter illustrates an input pulse 30 of duration t and having a centroid in time occurring after the initiation of the leading edge there-of at a time one-half 1 The waveform of the voltage v at point 11 of FIG. 1 is shown in general in FIG. 1a. The leading edge of input pulse 30 appears at time t at which time v drops from v to v At time t the input pulse trailing edge occurs. Between times t and t the RC discharge time constant is the product RC. When v finally rises to v current is supplied through diode 26 to transistor 20, causing the output pulse (at the collector of transistor 20) trailing edge to occur at time t Between times t and t the R-C discharge time constant is /2RC.

The centroid in time of the interval defined by the leading and trailing edges of input pulse 30 is and the output pulse trailing edge occurs at time t therefore, the following equation which will be proved states that the output trailing edge occurs at a fixed time after the centroid of the interval specified by the input pulse:

To prove Equation 1, the following R-C discharge equation is used:

('E- 2 v fi' ii- RC As can be seen in FIG. 1a, v is the value of v at time t From Equation 3b we can write The voltage v is the value of v at time t From Equation 30 we can write 2 l+( xi) By substituting v of Equation 4 into Equation 5, the following is obtained:

4 Equation 6 can be rearranged algebraically to become:

2t2t1t0 U31)1 (7) ln l where In denotes the natural logarithm. By multiplying both sides of Equation 7 by RC, then adding (t +t to both sides, and then dividing both sides by 2, the following equation is obtained:

0+: a i 2 2 l U2U1 Since R, C, v v and v;., are constants, the last term in Equation 8 is a constant. Let a constant time T be de- With Equation 9, the derived Equation 8 becomes Equation 1, the equation which was to be proved.

Illustrated in FIG. 3 is a complete circuit of the centroid finder of FIG. 1. In FIG. 3 all elements and components which are functionally similar to corresponding components of FIG. 1 are designated by the same reference numerals. The multivibrator action of the device of FIG. 3 is identical with that of FIG. 1. The timing capacitor and timing resistors 12 and 14 are also identical. In FIG. 3 the switch 16 comprises a diode 34 which is connected to be operated by a transistor 36 under the control of a zener diode 38 which in turn is controlled via resistors 40 and 46 by the input signal 30.

The input signal 30 is applied to the mutlivibrator at the base of transistor 18 via coupling capacitor 32, across resistor 42 and through diode 44. Capacitor 32 and resistor 42 comprise a differentiator whereby the positive going leading edge of input pulse 30 is utilized to trigger the multivibrator to its unstable state. The switch 34 is operated by the input pulse for the duration thereof.

The diod'e switch 34 is closed (i.e., it is conducting) when the input 30 is low. The diode 34 is allowed to conduct by reason of the fact that transistor 36 is cut off and its collector potential is high. The transistor 36 is cut off by lack of base current thereof which in turn is controlled by zener diode 38. The zener diode 38 is cut off when the input pulse 30 is low since the reverse potential applied across the diode 38 from the low input pulse 30 via resistors 40 and 46 to ground is not suificient to break down the reverse resistance of the zener diode.

When input pulse 30 goes high, the zener diode 38 breaks down to provide a positive base current to transistor 36 which then conducts to substantially ground its collector. The relatively low potential at the collector of transistor 36 back-biases diode switch 34 whereby the switch is open and remains open or non-conducting as long as the impulse 30 is high. Therefore, while the switch is open the capacitor 10 discharges solely through the relatively high impedance of resistor 14 alone.

When the input goes low again the zener diode 38 is cut otf, transistor 36 is cut off to forward bias diode switch 34 whereby the switch is closed and the capacitor 10 completes its discharge through the now halved impedance of resistors 12 and 14 in parallel. When the capacitor discharge is completed, current fiows from the source of positive potential through both resistors 12 and 14 and diode 26 through the base and emitter of transistor 20 to thereby initiate the switching action which returns the multivibrator to its stable condition.

Waveform b of FIG. 4 illustrates the potential at point b of FIG. 3 which controls the action of the diode switch 34. Waveform c of FIG. 4 (similar to the waveform of FIG. 1a) illustrates the potential at point 11, the side of capacitor 10 which is coupled with diode 26. As shown in FIG. 4, the capacitor discharges relatively slowly for the duration of the input pulse 30 during time interval t and thereafter discharges more rapidly until the end of the time interval one-half t -t-T. The output d at the collector of transistor is also illustrated in FIG. 4. Note that this output terminates to provide a negative going trailing edge at the time one-half t -l-T which is a fixed time interval T after the centroid in time of the time interval t The collector output d of the transistor 20 is differentiated by capacitor 50 and across diode 51 which may, if desired, have a resistor 52 connected in parallel therewith. Thus, the dilferentiator 50, 51, 52 provides the sharp or relatively short duration output pulse e of FIG. 4 which has a fixed chronological relation, time interval T, to the centroid in time of the input pulse 30.

As shown above, the interval T is a constant, independent of the time interval t provided only that T is equal to or greater than t The variation of the time constant of the timing circuit is most conveniently provided by the use of resistorsin parallel as illustrated in FIGS. 1 and 3. However, it will be readily appreciated, as illustrated in FIG. 2, that individual resistors of relative values R and one-half R may be utilized alternatively by the operation of the timing switch 16' or, of course, other arrangements for elfecting the desired change of the time constant of the circuit from a predetermined value to twice that value and then back to the predetermined value can be readily provided.

A modification of the centroid finder circuit of this in vention for use with a pulse shape correlating system, as described in a pending application of W. D. Ashcraft entitled Waveform Detector, Serial No. 841,550, filed September 22, 1959, and now Patent No. 3,095,541, is illustrated in FIG. 5. This circuit, as are the circuits of FIGS. 1 and 3, is essentially a modified one-shot or monostable multivibrator which is this case is triggered by the positive going leading edge of an input signal 39b appearing on trigger terminal 55. The circuit provides a short duration output pulse at a time interval T after the centroid in time of the duration of the controlling input pulse 30a which appears upon control input terminal 56. This circuit will provide a positive going signal at the end of the fixed time interval T in response to the negative going control input pulse 30a.

The centroid finder of FIG. 5 comprises a transistor 57 which is normally conducting by reason of base current supplied from a source of positive potential through a potentiometer 58, resistors 12 and 14, and diode 59 to the transistor base. Its collector is low and therefore the base of the second transistor 60 of the centroid finder is kept low by virtue of a conventional parallel resistancecapacitance collector-to-base coupling network 61. Thus the collector of transistor 60 is high to back-bias output diode 62 and provides a high potential at the circuit output terminal 63. The trigger and pulse input signals 30b and 30a are initiated at the same point in time. In fact, the trigger signal 30b which is of much greater duration than the pulse 300: may be provided from a conventional one shot or monostable multivibrator 45 which itself is triggered by the leading edge of control pulse 30a produced by pulse source 47. The pulse source 47 may be the transducer of an energy tracking system such as, for example, the photocell of an optical system. Of course, any source of signal specifying the uncertain time interval may be used.

The positive going leading edge of trigger 30b is applied via capacitor 64 to the base of transistor 60 to cause multivibrator switching action to occur and thus place transistor 60 in saturation and cause transistor 57 to be cut off whereby the output at point 63 goes low. The switching from stable to unstable condition is effected by the coupling of the negative going signal from the collector of transistor 60 via diode 65, timing capacitor 10, and switch 66 to cut off diode 59 and remove the positive potential from the base of transistor 57. The switch 66 may be utilized for purposes more particularly described in the aforesaid pending application to alternatively switch into the timing circuit timing capacitors 10a and 10b having different values than capacitor 10 so as to vary the timing of the multivibrator if deemed necessary or desirable.

As previously described, the timing capacitor 10 is provided with two possible discharge paths, both of which include the relatively small variable resistance 58 in this embodiment. One of these discharge paths includes resistor 14 while the other includes both resistors 14 and 12 which are of equal value as previously described. The two resistors are connected in parallel by means of the diode switch 26.

If the diode switch 26 is forward biased and conducts, both resistors 12 and 14 will be in the discharge path of the timing capacitor 10. If the switching diode 26 is biased to cut off, resistor 12 is eliminated from the discharge path, and the time constant is substantially doubled.

The negative going control pulse 30a, which by its leading and trailing edges specifies the time interval whose centroid is to be indicated, is applied to control the diode switch 26 by means of an input diode 70. When the input 30a is high, diode 70 is back-biased and switching diode 26 conducts whereby the switch is closed. When the input 30a is low, diode 70 conducts to back-bias switching diode 26 whereby the switch is open and resistor 12 is eliminated from the capacitor discharge path. Thus, it will be seen that the input 30a becomes low when the circuit is triggered by the positive going signal 30b and changes from low to high before the output pulse from the centroid finder at terminal 63 terminates. This change of the input 30a, before the termination of the output at 63, will take place whenever the input pulse duration is less than the greater of the two possible periods of the centroid finder circuit.

In some situations such as those more particularly pointed out in the aforesaid co-pending application it may be desirable to choose an input pulse of a certain maximum duration and to reject all input pulses which are greater than such maximum duration. The centroid finder circuit of this invention can be readily adapted to perform this second function in addition to its primary function of designating the centroid of the input pulse. The time interval T is one-half of the long time constant of the disclosed centroid finder circuit whereby it can be seen that if the input pulse has a duration equal to or greater than 2T the negative going output pulse of the centroid finder circuit of FIG. 5 will have a duration of 2T. If the negative imput pulse 30a has a duration less than 2T, it will be high when the centroid finder output again goes high. On the other hand, if the input 30a is of a duration greater than 2T, it will still be low when the centroid finder output goes high. Thus, the input pulse from pulse source 47 may be fed as one input to an And gate 49 and the centroid finder output from terminal 63 may be sharpened in differentiator 48 and fed as a second input to the And gate. With this arrangement the And gate will produce a high level output pulse only for those input pulses which are of duration equal to or less than 2T. It will be seen that such output of the And gate for input pulses of duration less than the interval 2T will occur at the fixed time interval T after the centroid in time of the input pulse 30a. For those input pulses 30a of durdation greater than 2T the And gate will produce no output since the positive going difierentiated trailing edge of the centroid finder output occurs when the input 30a is still low. Therefore, input pulses of duration greater than 2T are rejected.

It will be seen that there has been described a novel circuit wherein the time constant of a timing circuit is varied in accordance with the duration of an input signal which specifies the time interval of which the centroid is to be indicated. The novel operation is achieved by providing a first relatively large time constant for the duration of the specified interval and upon termination of the interval causing such time constant to be halved whereby the action of the timing circuit (in this case holding the multivibrator in its unstable condition) is 7 terminated at a time which has a fixed chronological relation to the centroid in time of the input pulse.

Although this invention has been described and illus trated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. In a monostable multivibrator having a pair of histate switching devices and means interconnecting said devices to have mutually exclusive states, said interconnecting means having a resistance-capacitance timing circuit of a first time constant, said multivibrator further having an input terminal for receiving a trigger pulse, the improvement comprising means for providing said multivibrator with a second timing circuit of different time constant, switch means for effecting alternative control of said multivibrator timing by one or the other of said circuits, and means for operating said switch means in accordance with the duration of said trigger pulse.

2. In a monostable multivibrator, a pair of bi-state switching devices interconnected to have mutually exclusive states, an input terminal for receiving a trigger pulse, a timing capacitor connected to be charged upon triggering of said multivibrator into its unstable condition, said capacitor being connected to shift said multivibrator back to its stable condition upon discharge of the capacitor, and means for controlling the discharge time of said capacitor in accordance with the duration of said trigger pulse.

3. Monostable apparatus comprising a pair of bi-state switching devices, means interconnecting said devices to have mutually exclusive states, said means including a timing capacitor and a discharge circuit therefor, said discharge circuit comprising first and second equal value resistors and a switching device interconnecting said resistors in parallel, whereby said apparatus has a first time constant when said switching device is closed and a second time constant when said switching device is open, said second time constant being twice said first time constant, means for triggering said apparatus in response to an input pulse, and means for operating said switching device for the duration of said input pulse.

References Cited by the Examiner UNITED STATES PATENTS 2,144,779 1/1939 Schlesinger 331-179 2,837,663 6/1958 Walz 30788.5 2,903,584 9/1959 Jaffee et al. 328 2,927,268 3/1960 Haggai et al 30788.5 2,945,966 7/1960 Davenport 307-88.5 2,955,255 10/1960 Van Winkle 328-185 2,986,649 5/1961 Wray 30788.5 3,033,998 5/1962 Nellis 30788.5

DAVID J. GALVIN, Primary Examiner.

GEORGE N. WESTBY, JOHN W. HUCKERT,

Examiners. 

1. IN A MONOSTABLE MULTIVIBRATOR HAVING A PAIR OF BISTATE SWITCHING DEVICE AND MEANS INTERCONNECTING SAID DEVICES TO HAVE MUTUALLY EXCLUSIVE STATES, SAID INTERCONNECTING MEANS HAVING A RESISTANCE-CAPACITANCE TIMING CIRCUIT OF A FIRST TIME CONSTANT, SAID MULTIVIBRATOR FURTHER HAVING AN INPUT TERMINAL FOR RECEIVING A TRIGGER PULSE, THE IMPROVEMENT COMPRISING MEANS FOR PROVIDING SAID MULTIVIBRATOR WITH A SECOND TIMING CIRCUIT OF DIFFERENT 